1.Bacteria, which live in the soil and on plant roots called legumes, convert nitrogen gas
into ammonia in a process known as nitrogen fixation.Other bacteria in the soil convert ammonia into nitrites and nitrates.

2.Producers can use them to make proteins.

3.Consumers then eat the producers and reuse the nitrogen to make their own proteins.

6.Other soil bacteria convert nitrates into nitrogen gas in a process called denitrification.

7.By this process nitrogen is released into the atmosphere again.

The Phosphorus Cycle

¨Phosphorus is important to living organisms because it forms part
of DNA and RNA.

¨It is not very common in the biosphere and remains mostly on land
in rock and soil minerals, and in ocean sediments.

¨Phosphorus exists in the form of inorganic phosphate and as the
rocks wear down, phosphate washes into rivers and streams where it dissolves and then eventually makes it way to the oceans.

¨How does this cycle work?

1.The phosphate that stays on land cycles between

organisms and the soil.

2.Plants absorb phosphate from the soil or from water and bind the phosphate into organic
compounds.

3.Organic phosphate moves through the food web, from producers to consumers, and to the rest
of the ecosystem.

Nutrient Limitation

¨Primary productivity is the rate at which organic matter is created by producers.

¨The amount of available nutrients controls the primary productivity
of an ecosystem.

¨If a nutrient is in short supply, it will limit an organism’s
growth.

¨A limiting nutrient is a single nutrient that is scarce or cycles very slowly.

¨Farmers apply fertilizers to their crops to boost their productivity.

¨Fertilizers contain three important nutrients:

Ønitrogen

Øphosphorus

Øpotassium

¨These nutrients help plants grow larger and more quickly.

¨Areas where limiting nutrient occur:

vopen oceans – nitrogen

voceans – iron and silica

vstreams, lakes and freshwater environments – phosphorus

¨When an aquatic system receives a large amount of a limiting factor
– runoff from a heavily fertilized field – an immediate increase in the amount of algae and other producers occurs
resulting in an algal
bloom.

¨The producers grow and reproduce quickly and if there are not enough
consumers to eat the excess algae, the surface of the water may become covered with algae.